A study of the initiation of yielding in mild steel

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Abstract

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This thesis presents the results of an experimental investigation of the behavior of an annealed low-carbon steel subjected to rapidly applied constant stress and to repeated short duration stress-pulses. The magnitude of the stresses in the short duration stress-pulses was greater than the upper yield stress. The material was aged at various temperatures between stress-pulses, and the effect of the time of aging on the number of stress-pulses to induce yielding was determined.

Plastic and anelastic microstrain of 4 x 10[superscript -6] in./in. to 37 x 10[superscript -6] in./in. is observed prior to the onset of yielding in rapidly applied constant stress tests and in repeated stress-pulse tests. The relation between equilibrium microstrain and stress is determined for stresses below the static upper yield stress. Aging of the specimens for a sufficient length of time at a given temperature between stress-pulses induces recovery in the material such that yielding does not occur in repeated stress-pulse and aging cycles. The activation energy of the recovery process corresponds, within the limits of the experimental accuracy, to the activation energies of carbon and nitrogen diffusion in the material.

These effects are discussed in terms of the dislocation theory of yielding. The recovery process is attributed to the diffusion of carbon and nitrogen to dislocations which have been displaced, thus stabilizing the array of dislocations for the particular stress condition.

Equilibrium microstrain produced by a particular configuration of dislocations is theoretically investigated. An electrostatic analogy of the dislocation model is used in estimating the microstrain. Comparison of the theory and the experimental data leads to the determination of a characteristic length of dislocation which agrees with previous concepts of a mosaic block structure. The delayed yield phenomenon may be qualitatively described by the action of the theoretical dislocation model.